On Mar 12, 2:02*pm, "Josepi" wrote:
Careful. Variacs are known waveform distorters. The government would not let
us use them for precision power measurements as the waveform adopts many
harmonics in some units.


Which brings up a couple of points. The temperature of the filament
in vacuum tubes does vary if you have AC applied to it. So you will
get some distortion from that. But most of the better DVM's are true
RMS meters, at least on Voltage. I would suspect they would also
measure true RMS current. But does anyone know for sure?

Dan